1. Field of the Invention
This invention relates generally to optical position sensing and more specifically to an improved signal amplifier for a solid-state position sensitive detector.
2. Description of the Background Art
Accurate position sensing is needed in such diverse fields as robotics and disk drives. Typically the problem arises in automatic control of structures, when an element of the structure is moved by some kind of motor. Servo systems compare the desired position of the structure with the measured position of the structure, and using this difference information supply more or less power to the motor. For this kind of servo system to work the measured position must be known to great accuracy.
Optical measuring systems are attractive in servo controlled systems because they do not introduce friction into the systems. Such friction could negate carefully designed critically-damped systems and cause either slow response or oscillation about the desired position. A typical prior art optical measuring system is shown in FIG. 1. Here the rotational position of arm 110 about pivot 112 is to be measured by light supplied by light source 100. Light source 100 may be a laser or some other collimated light source. The incident beam 104 from light source 100 is reflected by reflector 114. Reflector 114 is shown as a mirror but alternatively may be a beam splitter. As arm 110 pivots about pivot 112, angle A changes and the reflected beam 106 traverses a series of photodiodes 120 through 136. Depending upon which photodiode 120-136 is illuminated by reflected beam 106, the angle A of arm 110 is approximately known.
The device shown in FIG. 1 has the drawback of low positional resolution. The position of arm 110 is known only to a resolution depending upon the size and spacing of the photodiodes 120-136. Smaller photodiodes which are more closely spaced will yield higher resolution, but there is a limit to the practical size and spacing of discrete photodiodes. In addition, each photodiode has an anode and a cathode lead. Biasing and sensing a large number of individual photodiodes adds unwelcome complexity to the device.
A special kind of photodiode called a position sensitive detector (PSD) offers improved resolution and accuracy over the use of many discrete photodiodes. FIG. 2 shows the device of FIG. 1 where the individual photodiodes 120-136 have been replaced by a single PSD 210. The PSD 210 is a photodiode with an anode of width L. The PSD 210 has a common cathode 212 and a pair of anode connections, anode A 214 and anode B 216, attached at opposite ends of the anode of width L. The distance x from the center of PSD 210 of an illuminating spot produced by reflected beam 106 may be calculated by measuring the relative currents flowing in anode A 214 and anode B 216. The continuous anode of the PSD allows measuring resolution and accuracy to 1 part in 10,000 if coupled to a sensing amplifier of sufficient accuracy.
FIG. 3 shows a schematic symbol for a PSD. PSD 300 comprises a common cathode attachment 310 and a pair of anode attachments, anode A 312 and anode B 314. The schematic symbol for the PSD 300 also shows a schematic representation of incident light 316. When the PSD is reverse biased with voltage Vcc, a current Io flows depending upon the intensity of the incident light.
PSD 300 has the property that Io is dependent only on the intensity of the incident light and not on its position along the long anode. Currents IA and IB flow in anode A 312 and anode B 314, respectively. By current junction law, IO=IA+IB, and therefore the sum (IA+IB) is also dependent only on the intensity of the incident light and not on its position.
In FIG. 4 a schematic diagram for a prior art sensing amplifier and servo driver circuit using Gilbert cells is shown. The use of the 2-quadrant Gilbert cell for performing analog multiplications and divisions is well known in the art. In the FIG. 4 schematic, PSD is connected to an integrated circuit model AD880 (402), containing the Gilbert cells, manufactured by Analog Devices, Inc. The AD880 (402) has a sum output node 404 which may be used for laser power control, and a normalized difference servo output node 406 which yields the relative distance from the center of PSD 400 (as shown for PSD 210 in FIG. 2). In the FIG. 4 application, the position information is used for servo control over that position. The desired position is entered as digital data 412 into an inverted-output digital-to-analog converter (DAC) 410. The inverted analog output 414 of the DAC 410 is added to the non-inverted signal from the normalized servo output node 406 by lead/lag compensation circuit 420. Lead/lag compensation circuit 420 contains a summing operational amplifier (op amp) 422 whose output is zero if the measured position is the same as the desired position, and gives a correction signal otherwise. The output of lead/lag compensation circuit 420 is the input of servo control circuit 440. Servo control circuit 440 drives the arm control motor 450 in proportion to the correction signal from lead/lag compensation circuit 420. Servo control circuit 440 contains a current source op amp 442 and a current sink op amp 444 whose outputs at the current source node 446 and current sink node 448 send currents through the windings of arm control motor 450, keeping the arm in the desired position which was entered as digital data 412.
The primary shortcoming of the prior art circuit of FIG. 4 is the error induced by the analog divisions performed by AD880 (402). The observed error with this circuit is 1 part in 100, far below the 1 part in 10,000 intrinsic to the PSD 400.
In FIG. 5 a schematic diagram for a prior art sensing amplifier using gain control on the incident laser power is shown. In the FIG. 5 circuit the need for analog division is removed by controlling the illumination intensity from the laser 502 incident upon PSD 500. If the illumination intensity incident upon PSD 500 is a constant, then the sum of the anode currents (IA+IB) will be a constant, eliminating the need for normalization and the analog division errors induced thereby. The FIG. 5 circuit converts the PSD 500 anode currents into voltages with A buffer op amp 510 and B buffer op amp 512. The signals at voltage A node 514 and voltage B node 516 are added with analog adder 520 to yield a signal at A+B node 532 proportional to the incident intensity on PSD 500. Using this signal on A+B node 532, automatic gain control (AGC) circuit 534 sends a signal on AGC node 536 which adjusts the laser power controller 504 and thereby the laser 502 power output.
In other aspects, the circuit of FIG. 5 is equivalent to that of FIG. 4. The desired position is entered as digital input 528 to DAC 526, producing a desired position voltage on the analog output 524 of DAC 526. The A and B signal voltages are subtracted in analog subtractor 518 and compared with the desired position voltage using analog subtractor 530. The output of subtractor 530 is the input to the servo control circuit (not shown) which is identical to the servo control circuit 440 of FIG. 4.
The AGC circuit 534 control over laser 502 power output holds the incident light intensity on PSD 500 constant so that error-inducing normalization by division is not required. The circuit of FIG. 5 eliminates much of the error induced in the circuit of FIG. 4, but with the newly added limitation that the laser 502 power cannot be varied for other system requirements. In many applications, the laser's power needs to vary over a wide range, such as in the case of a read/write optical or magneto-optical disk drive. In such a case the positions of head positioning arms or fibers in a fiber-optic switch need to be controlled to high accuracy, and preferably using the same laser light source used to read and write from the disk. Using a beam-splitter, part of the incident laser light may be used for reading and writing from the disk while a small amount is available for position sensing. But in a read/write optical or magneto-optical disk the ratio of laser intensity for reading versus writing may be 1 to 10.
Therefore there exists a need for a system and a method for amplifying the outputs of a PSD which eliminates the errors created in the circuit shown in FIG. 4 and which does not require the limitation on laser intensity required by the circuit shown in FIG. 5.